However, the opportunity costs for a PhD are very, very high, especially if the degree takes longer than planned.

True. I had planed to finish my CS PhD in 2016, but now I am still not finished

You do get paid but quite little. I still took on debt during my physics PhD for lifestyle reasons (my significant other was 2k miles away, so flights were prioritized over savings).

I went to graduate school early 2000s. 6 years, average stipend < $15k and still had to pay student fees. Then a couple post docs for 5 more years at $40k.

There was significant opportunity cost, but now I've had 3 jobs that were all reasonably well paid that I only got because of the fanciness of my degree. I'm _not_ saying that's the right sorting for hiring managers to do, but it is what happened to me.

Net-net, I don't know if I'm even, behind, or ahead financially. I do know that graduate school was one of the best experiences of my life, my deepest friendships were formed in those years, and I miss pursuing my passion at that level constantly. So, I guess I'm in the position of not really caring if it lost me money.

Right I know, but I was making the assumption that they had a related undergrad, and more importantly, accounting for opportunity cost. Five years of working time for someone smart and quantitative enough to do a physics PhD is easily over six figures.